The present invention relates to a cooling method for devices which need to be actively cooled to function efficiently. Although not limited to such, the primary application for this invention is for the cooling of microelectronic systems, devices or chips, which generate relatively large quantities of heat during operation, and which need to be cooled to reduce the risk of malfunction.
The present invention provides cooling by the continuous supply and removal of a liquid coolant to the one or more surfaces to be cooled. The fluid is delivered in a finely atomized spray of liquid droplets, the spray forming a generally wide area distribution as it issues from the atomizer. Fluid is removed from the periphery of the surface by mechanical pumping. As will be made clear in the later description, the nature of the atomizers, atomized spray and fluid removal are key elements of the invention.
There have been numerous investigations and experiments into the capabilities of spray cooling. However, these investigations have, in most cases, been limited to a single, conventional atomizer that is commercially available. It will be shown that these nozzles are generally unsuitable for viable spray cooling. Additionally, these experiments have failed to address the many issues which must be resolved before spray cooling can be considered useful for the applications considered herein.
One set of experiments investigating the usefulness of spray cooling was conducted by one of the authors of the present invention, Donald E. Tilton, and the results of these experiments are summarized within a publication titled "Closed-System, High-Flux Evaporative spray Cooling", published in 1989. This research paper examined many of the variables involved in spray cooling, but did not implement a fully viable spray cooling system. Namely, the experiments relied upon the use of conventional atomizers, and did not provide for efficient fluid removal in a compact package. It does, however, establish the most advanced state of the prior art in the field of evaporative spray cooling.
Much of the commercial research and development of liquid cooling systems for microelectronics has been concentrated within the related field of jet impingement cooling, which consists of high velocity, narrow jets, or jet, of liquid directed upon the surface to be cooled.
Although the jet impingement method may appear nearly identical to spray cooling, particularly in that a liquid coolant is discharged from an orifice and directed at the cooled surface, there are many fundamental and significant differences in the fluid dynamics and heat transfer mechanisms between the impingement of a fluid jet, and the impingement of a well dispersed and finely atomized spray of liquid droplets over a much larger area. Jet impingement cooling has limitations which render it inherently inferior to spray cooling. It does not provide uniformity of cooling over the surface, requires higher flow rates for an equivalent average heat flux, and burns out (transition to vapor film boiling with consequent drastic increase in surface temperature) at lower critical heat fluxes than spray cooling.
The following description relates to FIG. 2, which depicts a liquid jet 20 cooling a heat source 21 in a phase change process. The highest heat flux is removed, as shown by the arrows, only in a very small area 22 directly under the jet in the stagnation region. After impinging upon the surface 23, the fluid 38 spreads radially away from the center of impingement. Two conditions are immediately obvious: one, the substantial momentum of the jet is no longer available to enable high heat flux. Specifically, the high efficiency cooling available from jet impingement occurs only upon the stagnation field directly under the center of the jet. Away from the center of the jet, with no appreciable momentum towards the surface 21, vapor 36 generated at the surface 21 can displace the liquid 38 from the surface, leading to burnout. Burnout occurs when vapor 37 completely displaces liquid 38. Two, the further the fluid travels radially from the center, the more the fluid slows down due to the fluid covering more area. Thus there is less tendency to displace hot liquid and vapor on the surface, and the heat flux must decrease. These radial variations in heat transfer directly result in surface temperature variations, and thermally induced stresses. This is highly undesirable with microelectronics, where such stresses are the leading cause of failure.
A further consequence of the uneven heat removal is a critical heat flux (CHF) lower than that achievable with spray cooling. When cooled by a jet, the outer region of an area transitions to film boiling (hence, where CHF occurs) at a relatively low heat flux, due to the lower heat transfer coefficients. This both reduces the heat removal in these areas, and increases the local surface temperature. This instantly places an incrementally larger heat removal burden on the inner areas, which cannot be accommodated because there is no corresponding incremental increase in heat transfer capability. Thus the film boiling phenomena quickly travels radially inward.
Closely packing jets to cool an area has two benefits in that the number of high flux areas are increased, and the total area cooled by each jet is decreased. These benefits are quickly offset by the geometrically increasing difficulty in delivering and removing the large quantities of fluid. As will be made clear in the description of the invention, the present invention does not suffer from these limitations.
Within the field of jet impingement cooling, several patents have been issued in which high flux cooling has been envisioned. One example is U.S. Pat. No. 4,108,242 to Searight et al. This patent addresses the issue of liquid, after contacting the heat source, rebounding off of the heat source and interfering with the original jet. Searight attempts to solve this issue by greatly increasing the volume of the system, so that the fluid has a place to go upon leaving the immediate area of the impinging jet. This solution is not very suitable for high flux cooling, and runs counter to compact packaging requirements. When one considers the total area of the heat exchanger within Searight's system, only a small portion is devoted to the high flux capability of the impinging jets.
U.S. Pat. No. 4,912,600 to Jaeger et al, another example of a jet impingement cooling system, also suffers the shortcomings mentioned in relation to FIG. 2. Also, the chips themselves are subjected to additional forces from the turbulent flow existing in the chamber, which could lead to chips detaching from the substrate after a period of time.
U.S. Pat. No. 4,838,041 to Bellows attempts to address the issue of fluid rebound/interference and vapor build-up within a jet impingement apparatus. Bellows uses a heat spreader for directing the fluid away from the area of the impinging jet. However, each spreader is several times larger than the microchip itself. Also, the spreader introduces an interfacial conduction resistance, which causes the chip to operate at a higher temperature for the same coolant temperature. The volume of the Bellows system remains relatively large due to the inherent problems associated with jet impingement cooling systems.
U.S. Pat. No. 3,844,343 to Burggraf has an appearance similar to the present invention. The heat transfer mechanism of the submerged jet single phase cooling is not nearly as efficient as evaporative spray cooling, however. Larger temperature differences and higher flow rates are required to gain the same cooling effect. Neither does this system have a fluid management component designed to maintain a thin liquid film on the heat source, as does the present invention.
U.S. Pat. Nos. 4,643,250 and 4,790,370 to Niggeman et al both attempt to describe an apparatus capable of providing evaporative cooling in the absence of gravity, or under adverse acceleration. U.S. Pat. No. 4,643,250 proposes to separate the liquid and vapor components of stored cryogen and use the liquid to condense the vapor. Since the quantity of liquid produced can only be less than the quantity of liquid thus vaporized, it is unclear why one would inefficiently condense the separated vapor to liquid instead of disposing of the vapor as waste. This question aside, the pertinent geometry which provides the jet impingement-assisted cooling is, for all practical purposes, identical to the previously mentioned U.S. Pat. No. 3,844,343 to Burggraf. Consequently, the Niggemann '250 system suffers from the same drawbacks and limitations as the Burgraff system. Additionally, the Niggemann '250 system suffers from the disadvantage of requiring a substantially increased operating pressure in order to provide sufficient pressure drop for the two sets of orifices in series. In the case of jetting a saturated fluid, this increased pressure is not available. With respect to the present invention this device is overly large for the amount of cooling provided.
The Niggemann '370 patent purports to provide an evaporative cooling system which can sustain momentary loss, or inversion of the gravity vector; gravity being what produces the buoyancy forces necessary for pool boiling to function. The means to achieve this is meant to be a pressurized non-condensable gas in conjunction with a blockage chamber. However, buoyancy forces are the result of gravity, or another acceleration, and not operating pressure. So, regardless of the system pressure, once boiling equilibrium is reached, if the module is inverted or subjected to "zero-g", buoyancy will be lost and the vapor bubbles growing on the surface will separate the liquid from the surface, causing burnout. Compared to the present invention, this evaporative cooling system is extremely large, capable of only 1/10th of the critical heat flux, and gravity dependent.
In contrast to prior art jet impingement cooling systems, prior art spray cooling systems have certain advantages, but with attendant problems associated with the viable application of spray cooling. FIG. 3 depicts a general cross-sectional view of spray cooling of a surface. A conical spray 30 impacts against a heat source 31 and forms a thin liquid film 32 which covers the entire surface 33 of the heat source. The heat flux arrows indicate a uniform heat removal across the entire surface 33. Uniform heat removal is important in reducing thermal stresses, and in delaying transition to CHF. The even heat removal is achieved by the uniform impingement of the liquid droplets over the entire area. The impinging droplets serve to mix the liquid on the surface, and to disrupt nucleating bubbles within the liquid film. Disrupting the growing vapor bubbles delays the transition to vapor film boiling (CHF), which occurs when liquid can no longer displace vapor from the surface. Keeping the liquid well mixed and in good contact with the cooled surface promotes heat removal by direct evaporation from the top of the liquid layer.
Although the uniform heat removal of a spray cooling system has definite advantages over the non-uniform heat removal of a jet impingement cooling system, there are attendant problems associated with spray cooling that have not been resolved in prior art spray cooling devices. As mentioned earlier, experimental spray cooling approaches have used a single conventional atomizer. A conventional atomizer must be spaced some inches from the heat source in order for the spray to fully develop prior to impacting against the surface to be cooled. In the field of electronics where miniaturization is critical, the size of a microchip (often less than 1/4 inch square) would be dwarfed by the distance needed for a spray to develop into a wide area distribution of droplets. Another problem associated with experimental prior art spray cooling systems, is that there have been no effective solutions proposed for removing the fluid from the system. Other problems associated with prior art spray cooling systems is that they have not addressed the issue of gravity dependence of the system nor the issue of vapor build-up within the system volume.
U.S. Pat. No. 4,352,392 to Eastman provides insight into prior art evaporative spray cooling. In this invention one can see the atomizer, spraying distance and system volume are all large with respect to the evaporator surface, or heat source. Further, there is no means for efficient removal of fluid. In the closed system embodiment depicted, the liquid and vapor are actually allowed to separate in the volume, and are removed separately. It can be appreciated that this system is inefficient, bulky, and gravity dependent.
U.S. Pat. No. 4,967,829 to Albers et al. is another example of prior art evaporative spray cooling. Once again, the high heat removal capability which is inherent in spray cooling is recognized, but the overall benefit available is not realized when the system volume is so large and inefficient. This system also requires very large distances to be traversed by the spray, leading to further inefficiency. Also, it relies on gravity to maintain the desired conditions within the volume.